I have a 25 Powerplay that will start without problems then within less than a minute the fuel flow gauge begins dancing wildly and the engine sounds like it is starving for fuel, then it dies. It will restart without problems. I changed the fuel/water seperator filter, removed the gas cap, but no change. It will run if I run at 2500 rpm but return to idle and it quits.
Could the electric fuel pump be bad. Sounds OK when you turn the key on....runs to pressurize then stops.
The engine is a 502 Mercruiser Mag with EFI.

Run a scan tool on the engine to see what codes are being thrown. It could just be an IAC.
Is this on a cold engine, hot or both?????

Is it a VST motor? The screen can get clogged on the pump never seen one bad enough to not idle but they can cause fuel starvation at higher rpm. If the screen has never been changed it should be. They were used from about 96-98

they have a anti drainback valve located on the outlet of the tank.it is a coast guard requirement.i have seen crap get stuck in the checkvalve and cause problems.try removing it and clean or replace it.

I agree w/ GRIFF get out the scan tool... IAC may be the issue might have some carbon build up or just a bad valve. Maybe many other issues also. A scan is in order here.
If it was a fuel issue it would get worse with demand, not better. it may be an oil pressure switch (I dont have the manual here) But most systems have a 2 sec prime with key on, and run during crank. Then run on the oil pressure switch once the motor starts. If the switch is going bad it might work at a higher oil pressure ie 2500 rpm, but at idle may be shutting off the pump or not allowing enough voltage to the pump. As a quick test hook up a multi meter to the pump and see if you are loosing voltage. or run the pump off the battery to see if it stays running. This sounds like it may be the issue here. I would have the get the manual out to verify the theory of operation on your motor to be sure it runs off the oil pressure switch during run. The older EFI motors, and carb engines with Electric pumps did. Now it is controlled off the computer and guardian modes. Not hardwired off the oil pressure switch during engine run anymore.
Hope this helps

I had the same problem on my previous boat. The fuel pick up tube was cracked at the fuel sender in the tank. It was not cracked off but it had a hairline crack and would suck air. Checking for codes is the proper thing to do but you have to be careful because it can cause false codes when it starves for fuel sometimes.

my buddy had the same thing go wrong on he boat . he change the fuel pump . his pump has low presure and a high presure line